In U.S. Pat. No. 4,642,080 of Hiroshi Takano et al., which patent is owned by the assignee hereof, a power transmission belt is disclosed having an elastomeric looped flat belt portion provided with a longitudinally extending tensile cord. A plurality of longitudinally spaced blocks are provided in aligned pairs on the outer and inner surfaces of the flat belt portion. The blocks are secured to the belt by bolts extending through the blocks and belt.
Such blocks are conventionally formed by winding fabric impregnated with a setting material, such as synthetic resin or rubber. Such blocks are made to be relatively hard and are provided with short fibers. One excellent form of known block utilizes glass fibers which provide excellent strength, high bending elasticity, compression resistance and wear resistance. Further, the glass fibers increase the frictional coefficient of the block against the belt surface so as to permit utilizing the friction between the block and the belt to resist shearing of the block from the belt when shearing forces are applied thereto, such as when the belt is used to transmit heavy loads with low belt tension.
One common form of such belt drive, however, is one utilizing speed shifting pulleys wherein the belt moves radially outwardly or inwardly about the pulley in effecting the desired speed change. Because of the relatively high friction of the fiberglass belt, however, resistance to smooth movement of the belt inwardly and outwardly between the two confronting faces of the pulley halves prevents a smooth transition in the speed change.
One attempted solution to this problem has been to mix oil MoS.sub.2 or graphite in the block material to reduce the coefficient of friction. Where oil has been used, it has been found that the volatile oil tends to leach out of the rubber during continued use, whereby the belt again has undesirable high friction characteristics.
In one form of known belt, the blocks are provided with recesses receiving outwardly extending projections on the belt for improved resistance to shear from the high loads applied thereto. It has been conventional to cause the recesses to accurately conform to the projections. As there is a tendency for the block portion engaging the neutral, or flat, belt portion at the sides of the projections to wear, the frictional engagement of the blocks with the flat belt portion thereat becomes reduced, thereby weakening the frictional retention force and thereby weakening the belt structure against shear forces applied to the blocks by high loads.